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Old September 6, 2012, 05:23 PM   #31
Join Date: March 4, 2005
Location: Ohio
Posts: 16,752
Have to agree there is some misinterpreting of information going on here that needs clarifying. Mr. Guffey's post makes the point that if you mind your geometric fundamentals, you'll stay out of trouble, but until you have all that clear in your head you can trip yourself up. He also reminds us there is more than one way to skin a cat. That's true of using the commercial gauge tools, too.

Several points to consider. First, if you are using the Hornady gauge and custom case gauge service, the reason they want a fired and un-resized case is because they want the neck loose enough for the bullet to slip in and out of it. They don't want the extra work of having to select and run a mandrel in that expands it. If the case was fired near full pressure in a high power rifle, it will be near the size of the chamber and using it as a gauge will tend to give you the longest COL (or shortest seating depth, whichever way you want to look at it). For that reason, I tend to think this service is mainly a convenience for people who are neck sizing only or who are careful not to bump their shoulders back more than a thousandth or two. Even then, all it does is save them a minor bit of measuring and calculation.

For cases resized more than the above, using that chamber sized case as a gauge can give you long COL (inadequate seating depth). I’ve already described seating to the difference between shoulder and ogive for cases that headspace on their shoulder. There’s a simpler way to skin that cat if you will accept a couple or three thousandths less precision.

As with that difference measurement, it frankly doesn't matter what gauge case you use as long as you know the difference between where it headspaces and where your resized cases headspace. If your cases resize with the headspace 0.005" longer, seating your ogives out 0.005" longer as measured by the bullet comparator (from the base of the head) will give you the same jump to the lands as the gauge case had, and vice versa. This is a couple or three thousandths less precise than the ogive to shoulder datum method owing only to the fact there is often that much variation in the length of cases coming out of a sizing die. But for most reloading it’s a second way to skin the jump control cat and do it well enough. Work as follows:
  1. Resize a bunch of cases.
  2. Measure the headspace datum distance from the back of the case head of at least fifteen (thirty is better) randomly selected ones.
  3. Average the results.
  4. Measure the headspace datum distance on your Hornady gauge case.
  5. Subtract the Hornady gauge headspace measurement from the average for your resized cases.
  6. Add the result to the bullet comparator length measurement you got using the Hornady gauge case.
Pay attention to the number sign doing this. If your cases are resized shorter than the Hornady gauge case, then you will have negative numbers, meaning you must seat that much shorter by the bullet comparator readings.

Note that above I referenced measuring headspace. The .300 Win Mag is designed to headspace on the belt. If you are carefully bumping shoulders back by small enough amounts that you actually headspace on the shoulder, then what goes for other cartridges that headspace on the shoulder will apply to it. But if you set the shoulder back far enough that you are actually headspacing on the belt, then two differences apply. One is that you need to measure headspace to the top end of the belt from the case head to get the comparison to the Hornady gauge case. The other is that once you have set the shoulder back far enough to start headspacing on the belt, setting it back still further won’t change the bullet comparator reading you want to use because it’s no longer affecting how far forward the neck can move in the chamber. The belt is then controlling that.

Just to clarify something mentioned earlier, a VLD's ogive is longer between the bullet tip and the place the ogive meets the throat than a normal bullet's is. It is therefore normal for VLD's to have longer-than-normal COL's when the ogive is the same distance off the lands as a standard spritzer nose it. That said, in many guns the Berger secant ogive VLD's like more jump to the lands than the more common tangent ogive bullets do. See this on seating them.
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Last edited by Unclenick; September 7, 2012 at 09:02 AM. Reason: typo fix
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